Tide

Topic: Movements of Ocean Water: Waves, Tides and Currents · NCERT: Class 11, Ch 13 "Movements of Ocean Water"

Meaning

A tide is the regular rise and fall of sea level, once or twice a day. It is caused mainly by the Moon's gravitational pull, and less by the Sun's pull, working against centrifugal force (the outward push felt by a body moving in a circle).

Tides are regular, so they can be predicted. This makes tide tables useful for ports, fishing, coastal building and disaster warning. In places with a large tidal range, tides can also be used to make tidal energy.

Explanation

How the tide-generating force makes two bulges

• Two forces act on the Earth:
• Gravity: the force by which one body pulls another. The Moon's gravity is the main cause of tides. The Sun is much bigger than the Moon, but it is much farther away, so its pull on the tides is weaker.

• Centrifugal force: the Earth and the Moon move around a shared centre, so the Earth feels an outward push. This push balances gravity.

• Tide-generating force is the difference between the Moon's pull and the centrifugal force. This difference is what makes tides.

• Near side (facing the Moon):
• This side is closer to the Moon, so the Moon's pull is stronger than the centrifugal force.

• Water is pulled towards the Moon, which makes the first bulge.

• Far side (away from the Moon):

• This side is farther from the Moon, so the Moon's pull is weaker.

• Here the centrifugal force is stronger, so water is pushed outwards. This makes the second bulge.

• Horizontal part matters more: the force has an up-down (vertical) part and a sideways (horizontal) part along the Earth's surface.

• The horizontal part slides water sideways and piles it up into the bulges (NCERT Figure 13.2).

The daily rhythm: why tides come about 50 minutes later each day

• Tidal bulge: a broad hump of ocean water raised by the tide-generating force. There is always one bulge on the Moon side and one on the far side.
• Cause → process → result:
• The Earth turns on its axis.
• Each coast passes under both bulges, and through the two low "troughs" between them.

• So most coasts get two high tides and two low tides.

• The lunar day:

• The Moon moves in the same direction as the Earth turns. After one full turn, the Earth has to turn a little more to "catch up" with the Moon.
• So a lunar day (the time a place takes to face the Moon again) is 24 hours 50 minutes [5].
• Two high tides come 12 hours 25 minutes apart [5].
• So high tide comes about 50 minutes later each day.

Types of tides

By how often they come (in one lunar day):

Type	Pattern	Example area
Semidiurnal	2 highs + 2 lows, about equal in height	Much of the east coast of North America [6]
Mixed semidiurnal	2 highs + 2 lows, unequal in height (a higher high water and a lower high water)	Much of the west coast of North America [6]
Diurnal	1 high + 1 low	Many areas of the Gulf of Mexico [6]

By height (where the Sun is compared with the Moon):

• Spring tide (it has nothing to do with the season):
• The Sun, Moon and Earth are in a straight line, at new moon and full moon.

• Their pulls add up, so the tidal range is highest. These tides give the highest and lowest waters of each lunar month [5].

• Neap tide:

• The Sun and Moon are at 90° to each other as seen from the Earth, about midway between new moon and full moon.
• Their pulls partly cancel. The range is usually 10–30% less than the average range [5].

Tidal range: what makes tides high or low

• Tidal range: the difference in height between high water and low water.
• The Moon and the Sun set the basic rhythm, but the shape of the coast decides the height.
• Where the range is large:
• Wide continental shelves: water piles up over the shallow sea floor, so the bulge rises higher.

• Funnel-shaped bays and estuaries: an estuary is the wide mouth of a river where it meets the sea. Incoming water is squeezed into a narrower and narrower space, so it rises higher.

• Where the range is small: mid-ocean islands. The deep open ocean around them has no shelf or funnel to pile the water up.

• Tidal current: the flow of tide water when it is squeezed between islands or into bays and estuaries.
• Tidal bore: a wall of water that rushes upriver against the river's flow.
• A strong incoming tide, usually a spring tide, enters a narrowing, funnel-shaped estuary.
• The water is squeezed and cannot spread out, so the front of the tide becomes steep.
• The result is a moving wall of water that travels upstream.

Where it occurs

• World:
• Bay of Fundy, between Nova Scotia and New Brunswick, Canada (North Atlantic coast): the highest tides in the world, with a bulge of 15–16 m. There are two tides a day, so the water rises about 240 cm an hour. The bay is long and narrows like a funnel.
• Semidiurnal tides: much of the east coast of North America. Mixed semidiurnal tides: much of the west coast of North America. Diurnal tides: many areas of the Gulf of Mexico [6].

• Tidal bores: Qiantang (Qiantang River, China), the largest in the world; Pororoca (Amazon, Brazil), a local name meaning "great roar"; Severn bore (Severn, United Kingdom).

• India:

• Gulf of Khambhat (Cambay), Gujarat: the largest tidal range on India's coast, about 10–11 m at its head (the narrow inner end of the gulf). It is a funnel-shaped gulf on a wide shelf.
• Gulf of Kachchh (Kutch), Gujarat: the second largest range.
• Hooghly–Sundarbans delta, West Bengal: the river channels act as funnels, so the range is larger here than on the rest of the east coast. The Hooghly bore near Kolkata is a danger to boats and to Kolkata Port shipping.
• Rest of the east coast: small ranges.
• Tidal energy potential: about 8,000 MW in total (Ministry of Power, PIB, 2017). This includes Gulf of Khambhat ~7,000 MW, Gulf of Kachchh ~1,200 MW and the Gangetic delta, Sundarbans ~100 MW [2]. A later estimate puts the total identified potential at about 12,455 MW (PIB, 2019) [7].

Don't confuse with

• Storm surge: a cyclone's winds push sea water onto the coast. Surges come from wind and changes in air pressure at irregular times, so they are not tides. Tides are regular and can be predicted.
• Spring tide vs neap tide: a spring tide comes at new and full moon, when the Sun, Moon and Earth are in a line, and has the highest range. A neap tide comes when the Sun and Moon are at 90°, and its range is 10–30% less than average [5].
• Far-side bulge vs Sun's pull: the bulge on the side away from the Moon forms because centrifugal force is stronger than the Moon's pull there. The Sun's pull does not cause it.
• Tidal bore vs tidal current: a tidal current is the flow of tide water squeezed between islands or into bays. A tidal bore is a steep wall of water moving upriver against the river's flow.

Prelims Hooks

• The tide-generating force is the Moon's pull minus centrifugal force. Its horizontal part, not its vertical part, mainly builds the bulges.
• Lunar day = 24 h 50 min. High tides come 12 h 25 min apart, so each day's tide is about 50 minutes later [5].
• Spring tide: new moon and full moon (Sun, Moon and Earth in a line). Neap tide: Sun and Moon at 90° [5].
• Bay of Fundy (Nova Scotia–New Brunswick, Canada) has the world's highest tides, at 15–16 m.
• Tidal range in India, from highest to lowest: Gulf of Khambhat > Gulf of Kachchh > Hooghly–Sundarbans > rest of the east coast.
• Bore–river matches: Qiantang–China (largest), Pororoca–Amazon (Brazil), Severn–UK, Hooghly–India.

Mains Points

• Why tidal range differs from place to place (GS-I):
• The Moon and the Sun only set the rhythm. The shape of the coast decides the height.

• Wide shelves and funnel-shaped gulfs (Fundy, Khambhat, Kachchh) make the tide much higher. Mid-ocean islands and India's narrow-shelf east coast have small ranges.

• Tidal energy in India (GS-III):

• A barrage (a dam across a bay or estuary) traps water at high tide and lets it out through turbines at low tide. To get enough power, high tide must be at least 5 m higher than low tide. In India, only the Gulf of Cambay and the Gulf of Kutch have such sites [3][4].
• Problems: a high capital cost of about ₹30–60 crore per MW, so tidal power is not yet commercial [3][4]. Barrages can also harm the mangroves, mudflats and fisheries of the gulfs, and silt is a problem.

• Policy: in 2019, the Power Minister approved declaring ocean energy, including tidal energy, as Renewable Energy [7]. This lets such projects count towards renewable purchase obligations.

• Tides, ports and disasters (GS-III):

• Tidal currents and the Hooghly bore make sailing and dredging difficult at Kolkata Port, so ships depend on tide tables.
• A storm surge that arrives at high spring tide floods much more land, as seen in the Sundarbans and Odisha during cyclones. This is why tide data matters for coastal early warning and disaster management.

Related concepts

• Tide-generating force
• Tidal bulge
• Tidal range
• Tidal current
• Bay of Fundy
• Tidal bore

Read more

• Tides: why the sea rises and falls